An automatic analyzer for clinical tests dispenses predetermined amounts of a reagent and a sample for subsequent mixing and reaction. Absorbance of a reaction solution is then measured for a predetermined period of time and, based on measurements, concentration of substances to be measured are found.
The number of tests per hour is used as an index indicating processing capacity of apparatuses. Since the development of the automatic analyzers, a number of manufacturers of automatic analyzers have been making effort to develop means of increasing the processing speed of the apparatuses, in addition to enhanced accuracy of measurements. Efforts toward the increased processing speed of the apparatuses have been embodied in such areas as the increased number of reaction cells to be used (increased size of the apparatus), higher dispensing speed of probes for specimens and reagents (higher probe motions), faster and more efficient specimen rack transfer lines, and higher data processing speed of PCs. This has resulted in a substantially shorter time between blood sampling to measurements report. One of factors that limit the processing speed of measurement in current automatic analyzers that offer a higher throughput is time it takes a specimen and a reagent to react with each other, a reaction time, during measurement. The reaction time depends on reactivity of the reagent. The reaction time in biochemical analyzers is typically ten minutes per one item. Sample-and-reagent reaction end time varies greatly for each item and measurement methods of clinical tests can be classified into an endpoint assay and a rate assay according to the analysis method.
In the endpoint assay, a change in absorbance decreases with time and eventually approaches asymptotically a predetermined value (final absorbance). Concentration of the component to be measured in the sample is obtained from the value of absorbance which the change in absorbance asymptotically approaches. Even with the endpoint assay, the final absorbance is reached at relatively early stages with some items, such as T-CHO (total cholesterol) and Glu (glucose). While, with other items, such as CRE (creatinine), TP (total protein), CRP (C-reactive protein) by an immunoturbidimetric method, IgA (immunoglobulin A), IgG (immunoglobulin G), and IgM (immunoglobulin M), with which a reaction progresses only mildly and takes time to reach the final absorbance by way of a final steady state.
The rate assay is typically a test method that measures a speed at which a reaction progresses from a start of the reaction between a specimen and a reagent. A rate of change in absorbance of the rate assay substantially remains constant and the reaction process is linear. With an enzyme method as a type of the rate assay, the reaction continues until a substrate or a coenzyme is consumed, so that the absorbance continues increasing or decreasing and is not constant, except when the specimen concentration is so high as to exceed a permissible range. An activity value of the item is therefore calculated from the speed of the linear change in absorbance, and not the concentration of the enzyme itself. If the reaction stops within a measurement time for use in speed calculation and the absorbance undergoes a sudden change, and if the absorbance at that particular point is used, concentration of the item cannot be measured correctly. The reaction speed is therefore calculated without using the absorbance at that photometric point. In patent document 1, a calculation method that eliminates the need for dilution and repeat run may be employed.
As a method for obtaining favorable measurements even when a sufficiently long time cannot be made available for the reaction time, patent document 2, for example, discloses a method for approximating a relationship between absorbance and time with y=A+(B−A)/exp(Kt), using measured time and absorbance data and the least squares method. Where, A denotes final absorbance, B denotes reaction initial absorbance, K denotes a reaction speed constant, and t denotes measurement time. In this method, the concentration of the substance to be measured is obtained based on obtained A, B, and K.